1. Field of the Invention
The present invention relates to a brake fluid pressure controller for a vehicle, and particularly, to a brake fluid pressure controller for controlling driving of a pump configured to intensify brake fluid pressure supplied by a master cylinder.
2. Description of the Related Art
There has been known a brake fluid pressure controller for a vehicle which performs anti-lock breaking control, traction control, electronic brake force distribution and the like. Such a brake fluid pressure controller can supply brake fluid pressure (auxiliary pressure) to wheel cylinders not by operation of a brake pedal, but by a pump driven by a motor. Recently, reduction of operation noise of the pump has been demanded from the viewpoint of comfortable driving environment [for example, see Japanese unexamined patent publication Kokai 2000-203401 (paragraph 0038) (hereinbelow referred to as “Patent Document 1”), Japanese Unexamined Patent Publication Kohyo H6-505939 (claim 1 and page 3, right upper column, lines 2-3) (hereinbelow referred to as “Patent Document 2”) and Japanese unexamined patent publication Kokai 2001-39286 (claim 1, paragraph 0050, FIG. 6) (hereinbelow referred to as “Patent Document 3”)].
Patent Document 1 discloses a device which suppresses operation noise of a motor by driving the motor at high revolutions (duty ratio of 100%) for a predetermined period of time and driving the motor at low revolutions (low duty ratio) after the predetermined period of time.
In Patent Document 2, an attempt is made to reduce noise by operating a motor for driving a pump at 40-60% of a nominal speed, during traction control. In Patent Document 3, an attempt is made to reduce noise by delaying an initiation of pump operation when the vehicle is under certain conditions, specifically under conditions where brake force distribution is electronically conducted and the vehicle is neither in a high-speed driving state nor in a turning state, on non-low μ road.
To sum up, in these prior arts, attempts are made to reduce noise by driving the pump at low speed or delaying the initiation of pump operation, while retaining performance of various necessary functions at a sufficient level.
However, in a case of the device of Patent Document 1, the operation noise of the motor is not satisfactorily suppressed, and further suppression of the operation noise is demanded. In addition, in a case where a part of the brake fluid with pressure intensified by the pump is returned to the master cylinder, operation noise of a cut valve is frequently generated by a pulsation of the brake fluid returning to the cut valve.
By the way, there has also been known a vehicle stability control which controls an overall behavior of a vehicle including, in addition to anti-lock breaking control and traction control, sideslip suppression control where vehicle behavior is stabilized by controlling sideslip or spin when the vehicle is turning. Conventionally in vehicle stability control, when a difference between a target fluid pressure and an estimated fluid pressure becomes a predetermined value or more, it is judged that rapid elevation of pressure is required, and the motor is driven under higher power. However with this judgment method, the above-mentioned condition frequently occurs, i.e., the difference between the target fluid pressure and the estimated fluid pressure frequently becomes the predetermined value or more, and thus the motor is frequently driven at higher revolutions. As a result, noise due to operation had not been fully reduced.
Therefore, it is desired that the operation noise of the motor, the pump, the cut valve and the like be sufficiently reduced during brake fluid pressure control by the pump in behavior stabilization of the vehicle, while maintaining a pressure-intensifying performance of the motor. Especially in the case where overall behavior stabilization of the vehicle is controlled, it is desired that operation noise of the motor, the pump, the cut valve and the like be sufficiently reduced by setting and controlling the operation conditions of the pump based on a state of turning movement, since a fluid pressure required for stabilization varies depending on the state of turning movement.